Control of Precambrian Basement Deformation Zones on Emplacement of the Laramide Boulder Batholith and Butte Mining District, Montana, United States

Scientific Investigations Report 2011–5016

U.S. Department of the Interior U.S. Geological Survey Cover photo: Sheeted granite of Boulder batholith near Homestake Tunnel, Jefferson County, Montana. Circa 1900. (Photo by W.H. Weed; from U.S. Geological Survey Professional Paper 74, 1912, plate 2-B.) Control of Precambrian Basement Deformation Zones on Emplacement of the Laramide Boulder Batholith and Butte Mining District, Montana, United States

By Byron R. Berger, Thomas G. Hildenbrand, and J. Michael O’Neill

Scientific Investigations Report 2011–5016

U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior KEN SALAZAR, Secretary

U.S. Geological Survey Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2011

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Suggested citation: Berger, B.R., Hildenbrand, T.G., and O’Neill, J.M., 2011, Control of Precambrian basement deformation zones on emplacement of the Laramide Boulder batholith and Butte mining district, Montana, United States: U.S. Geological Survey Scientific Investigations Report 2011–5016, 29 p. iii

Contents

Abstract...... 1 Introduction...... 1 Paleoproterozoic Tectonics in Southwestern Montana...... 2 Mesoproterozoic Tectonics in Southwestern Montana...... 2 Late Cretaceous Tectonics in Southwestern Montana...... 2 The Boulder Batholith...... 4 Tectonic and Structural Localization of Early-Stage Magmas...... 4 Tectonic and Structural Localization of Later-Stage Magmas...... 10 Late-Stage Dikes and Veins...... 10 Geophysical Model of the Butte Quartz Monzonite...... 10 Geophysical Data and Modeling...... 12 Results of Geophysical Data Analysis...... 17 The Paleocene Butte Mining District...... 21 Discussion...... 23 Imprint of Proterozoic Deformation Fabrics on Boulder Batholith...... 23 Shape and Controls on Emplacement of the Boulder Batholith...... 23 Depth of Emplacement of Batholith...... 26 Butte Mining District...... 26 Conclusions...... 26 References...... 27

Figures

1. Precambrian basement terranes of the western United States showing the location of the Great Falls tectonic zone, the Butte mining district, and the Proterozoic Belt Basin...... 3 2. Upper intercept zircon dates from intrusive rocks in the Great Falls tectonic zone...... 4 3. Lineations, foliations, and orogenic gold veins in Archean rocks in the southernmost Tobacco Root Mountains...... 5 4. Outline of the Belt Basin on a part of the basement terrane...... 6 5. Generalized geologic map of a part of the Highland Mountains at the southern end of the Boulder batholith...... 6 6. Generalized tectonic map of southwestern Montana during the Late Cretaceous showing the principal thrust-fault complexes, regional-scale faults, and locations of the Proterozic Belt Basin and Cretaceous batholithic complexes...... 7 7. Generalized geologic map of the Boulder batholith...... 8 8. Generalized geologic map of a part of the east margin of the Boulder batholith...... 9 9. Topographic map showing linear trends in the regional gravity and magnetic data defined after removal of those correlating with topographic trends...... 11 iv

10. The distribution and orientation of early- and late-stage aplite, pegmatite, and alaskite dikes on a generalized geologic map of the northern half of the Boulder batholith...... 12 11. Rose diagram for late-stage dikes in the Butte Quartz Monzonite in the southern half of the batholith...... 13 12. The distribution and orientation of latest-stage sulfide-bearing quartz veins on a generalized geologic map of the northern half of the Boulder batholith...... 14 13. Reduced-to-pole aeromagnetic maps of the Boulder batholith and vicinity...... 15 14. Gravity maps of the Boulder batholith and vicinity...... 16 15. Simultaneous inversion of magnetic and gravity data across the Boulder batholith showing computed and actual magnetic and gravity fields and 2D model for profiles 1 and .5...... 18 16. Simultaneous inversion of magnetic and gravity data across the Boulder batholith showing computed and actual magnetic and gravity fields and 2D model for profiles 3 and .8...... 19 17. Contour map of calculated depths to the base of the Butte Quartz Monzonite using a 3D gravity inversion model...... 20 18. Map of polymetallic quartz veins and faults in the Butte mining district...... 22 19. Structural model of the origin of the faults and fractures that localized veins in the Butte mining district...... 24 v

Abbreviations Used in This Report

2D two-dimensional 3D three-dimensional A Avon, Montana A/m ampere-turn per meter Ag silver AN Anaconda mine AR Archean rocks As arsenic BB Boulder batholith BH Big Hole Valley BP Burton Park pluton CG Climax Gulch pluton Cu copper D Donald pluton DL Deer Lodge Valley FS Fleecer Stock Ga giga-annum GC Clark Fork River valley GFTZ Great Falls tectonic zone GL Garnet line HC Hell Canyon pluton HM Highland Mountains IB Idaho batholith IGSN International Gravity Standardization Net K Kelley mine kg/m3 kilogram per cubic meter km kilometer KR Kokoruda Ranch Complex kt kiloton L2 Leonard No. 2 mine LT Lombard thrust LX Lexington mine M magnetic susceptibility m meter m.y. million year (duration) Ma mega-annum (point in time) vi

MC Moose Creek pluton mG milligal MN Mountain Consolidated mine Moz megaounce Mt megaton nT nanotesla P Proterozoic Belt Basin rocks Pb lead PB Pioneer batholith PL Perry line RC Rader Creek Granodiorite pluton S Steward mine Sr strontium SWMTZ southwest Montana tectonic zone SWS Silver Wave stock TR, TB Tobacco Root batholith TRM Tobacco Root Mountains U Unionville Granodiorite pluton U uranium Zn zinc ρ density Control of Precambrian Basement Deformation Zones on Emplacement of the Laramide Boulder Batholith and Butte Mining District, Montana, United States

By Byron R. Berger, Thomas G. Hildenbrand, and J. Michael O’Neill

Abstract fabrics found outside the district. The faults controlling the Butte veins are interpreted to have formed through activation What are the roles of deep Precambrian basement under shear of preexisting northeast-striking joints as master deformation zones in the localization of subsequent shallow- faults from which splay faults formed along generally east- crustal deformation zones and magmas? The Paleoproterozoic west and northwest joint plane orientations. Great Falls tectonic zone and its included Boulder batholith (Montana, United States) provide an opportunity to examine the importance of inherited deformation fabrics in batholith Introduction emplacement and the localization of magmatic-hydrothermal mineral deposits. Northeast-trending deformation fabrics The reactivation of Precambrian deep-crustal deformation predominate in the Great Falls tectonic zone, which formed zones, often bounding tectonic terranes, has long been hypoth- during the suturing of Paleoproterozoic and Archean cratonic esized to play a role in subsequent shallow-crustal faulting, masses approximately 1,800 mega-annum (Ma). Subsequent the emplacement of plutons, and the structural localization Mesoproterozoic to Neoproterozoic deformation fabrics trend of magmatic-hydrothermal mineral deposits (for example, northwest. Following Paleozoic through Early Cretaceous Billingsley and Locke, 1941). Although multiple studies have sedimentation, a Late Cretaceous fold-and-thrust belt with demonstrated a spatial correlation between basement defor- associated strike-slip faulting developed across the region, mation zones and mining districts (for example, Grauch and wherein some Proterozoic faults localized thrust faulting, others, 2003; O’Neill and others, 2004; Emsbo and others, while others were reactivated as strike-slip faults. The 81- to 2006), few studies relate basement deformation fabrics to the 76-Ma Boulder batholith was emplaced along the reactivated structural control on mineralized plutonic rock complexes. In central Paleoproterozoic suture in the Great Falls tectonic this report we use potential-field geophysical data to interpret zone. Early-stage Boulder batholith plutons were emplaced the structural control on plutons, dikes, and mineral deposits in concurrent with east-directed thrust faulting and localized the Late Cretaceous Boulder batholith, Montana, in the context primarily by northwest-trending strike-slip and related faults. of Precambrian basement deformation zones. We also inter- The late-stage Butte Quartz Monzonite pluton was local- pret the structural controls on postbatholith Paleocene dike ized in a northeast-trending pull-apart structure that formed emplacement and mineralized veins in the batholith-hosted behind the active thrust front and is axially symmetric across Butte mining district. the underlying northeast-striking Paleoproterozoic fault zone, interpreted as a crustal suture. The modeling of potential-field The discussion covers the following: (1) the predomi- geophysical data indicates that pull-apart–stage magmas fed nant deformation fabrics and their evolution in Proterozoic into the structure through two funnel-shaped zones beneath the basement rocks in southwest Montana; (2) the evidence for batholith. Renewed magmatic activity in the southern feeder the reactivation of many Precambrian structures during Late from 66 to 64 Ma led to the formation of two small porphyry- Cretaceous deformation in southwest Montana; (3) a proposed style copper-molybdenum deposits and ensuing world-class two-stage model for the structural controls on the emplace- polymetallic copper- and silver-bearing veins in the Butte min- ment of the Boulder batholith; and (4) a new model for the ing district. Vein orientations parallel joints in the Butte Quartz structural evolution of the vein systems in the Butte mining Monzonite that, in turn, mimic Precambrian deformation district. 2 Control of Precambrian Basement Deformation Zones on Emplacement of the Laramide Boulder Batholith, Montana

Paleoproterozoic Tectonics in embayment, systematic changes in the thicknesses of basin sediments are suggestive of the importance of east-west faults Southwestern Montana with normal-sense displacement (Winston, 1986). The spatial correlation of east-west Mesoproterozoic faulting with Paleo- The collision of an Archean terrane and a Paleoprotero- proterozoic east-west foliation in the Highland and northern zoic terrane about 1,800 mega-annum (Ma) created a north- Tobacco Root Mountains is suggestive of a Paleoproterozoic east-trending deformation zone 1,500 kilometers (km) long ancestry to the Helena embayment. by as much as 200 km wide known as the Great Falls tectonic zone (O’Neill and Lopez, 1985) (fig. 1). An abrupt change across this belt in the antiquity of upper intercept dates of Late Cretaceous Tectonics in uranium-lead (U-Pb) zircon ages (fig. 2) and a belt of volcanic and plutonic rocks led O’Neill (1998) to postulate that the Southwestern Montana principal fault boundary or suture underlies the central axis of the tectonic zone. Beginning in the Late Cretaceous and continuing into Ductile and brittle deformation, igneous intrusive activity, the early Eocene, a period known as the Laramide, there was and metamorphism characterize the Great Falls tectonic zone significant tectonic and magmatic activity in western Mon- deformation (O’Neill, 1998). Collision-related deformation tana (fig. 6). East- to northeast-directed thrust faults that are fabrics are well exposed in mountain ranges at the southern younger to the east accommodated east-west compression. The end of the Late Cretaceous Boulder batholith (fig. 2), in what thrust front north of the Boulder batholith, labeled the Mon- is known as the Highland Mountains, and farther southeast tana disturbed belt on figure 6, corresponds to the northeast in the Tobacco Root Mountains that host the Tobacco Root edge of the Proterozoic Belt Basin. The coincidence of the batholith (fig. 2). In the Highland Mountains the deforma- basin edge and frontal thrusts is suggestive that a northwest- tion is complex, but, overall, foliation strikes northeast to trending tectonic inheritance guided the frontal thrust zone east-northeast. Paleoproterozoic mafic dikes and sills intruded immediately east of the Boulder batholith. subparallel to the foliation. To the east and southeast in East of the Boulder batholith, the north and south normal- the Tobacco Root Mountains, directly east of the Highland fault boundaries of the Helena embayment (fig. 4) were Mountains, foliation and dikes trend east-northeast and west- reactivated as strike-slip and reverse-slip fault zones to form northwest and, farther south, trend northeast. From such map the Helena structural salient of the frontal thrust zone (fig. 6). data, we infer that the regionally predominant, underlying These transverse zones consist of anastomosing systems of Paleoproterozoic fabric in the Archean rocks trends northeast faults (see Schmidt and O’Neill, 1982). Within the salient, parallel to the main tectonic suture, except in the Highland northwest-striking strike-slip faults accommodated shortening and northern Tobacco Root Mountains, where a generally strains west of the thrust front. east-west fabric dominates. Figure 3 illustrates the character On the foreland southeast of the Boulder batholith and of this likely regional pattern in an area at the south end of the south of the Willow Creek fault zone, the principal mechanism Tobacco Root Mountains. of shortening east of the thrust front was northwest-striking left-reverse (oblique) faults (fig. 5). Schmidt and Garihan (1986) documented a significant difference in the offset of Mesoproterozoic Tectonics in Archean marker units relative to overlying Paleozoic units, Southwestern Montana indicating the northwest faults are Meso- to Neoproterozoic faults that have been reactivated. A northwest-trending depositional trough, referred to Initially, thrust faulting with very local strike-slip dis- as the Belt Basin (for example, Harrison, 1972), began to placements was the primary mechanism for accommodating form across the Great Falls tectonic zone at approximately Late Cretaceous regional strains. As the thrust front advanced 1,500 Ma (fig. 4). The northeast edge of the basin trends farther east, strain accommodation behind it was predomi- northwest, parallel to the basin axis, and may have a structural nantly on northwest-striking strike-slip faults. North and root. Northwest-striking faults with Proterozoic ancestry—the northwest of the Boulder batholith, where Cretaceous plutonic Lewis and Clark line—are exposed in the west-central (see rocks are scarce, there is an approximately 440-km-long zone Harrison, 1972) and southeast parts of the basin (see Schmidt of strike-slip and related faulting known as the Lewis and and Garihan, 1986). Mesoproterozoic and Neoproterozoic Clark line (fig. 6). The Neoproterozoic age and intermittent diabase dikes are within or parallel to the faults (Schmidt and Phanerozoic reactivation of some faults in the Lewis and Clark Garihan, 1986). Figure 5 shows the orientations of dikes in line were documented by Hobbs and others (1965) near its the Highland Mountains and nearby Tobacco Root Mountains. western end, and Reynolds (1979) documented activity of a In the southeast part of the basin in what is called the Helena similar age at the eastern end. Late Cretaceous Tectonics in Southwestern Montana 3

115° 105°

HEARNE Area of figures 2 & 4 (2.7-1.8 Ga)

tructure an s ulc TRANS-HUDSON V (3.2-2.5 Ga) 50° MEDICINE HAT CANADA (3.3-2.6 Ga) UNITED STATES PRIEST RIVER  GFTZ SUPERIOR (>2.5 Ma) Butte   ND SD

 MT 45° WY (2.4-1.6SELWAY Ga) 

 WYOMING  

 (>2.5 Ga)   ID NV  GROUSE  ne Belt  CREEK  Cheyen NE   (>2.5 Ga)  CO KS    40° 

MOJAVE YAVAPAI (2.5-1.2 Ga) (1.9-1.7 Ga)    UT  OK

AZ 

 NM TX



35° 

  MAZATZAL 

  (1.8-1.6 Ga)   GRENVILLE

 (1.4-1.0 Ga)

MEXICO  

0 200 KILOMETERS

0 20 MILES

EXPLANATION Proterozoic Belt Supergroup rocks (after Foster and others, 2006) Tectonic boundary of Precambrian basement terrane—Dashed where inferred  Thrust-faulted tectonic boundary of Precambrian basement terrane—Dashed where inferred Approximate tectonic boundary between unexposed Archean (to southeast) and Proterozoic (to northwest) terranes in Great Falls tectonic zone (GFTZ) Approximate boundary between 87Sr/86Sr<0.706 in intrusive rocks to the west and 87Sr/86Sr>0.0706 to the east

Figure 1. Precambrian basement terranes of the western United States showing the location of the Great Falls tectonic zone (GFTZ), the Butte mining district (star symbol), and the Proterozoic Belt Basin. (Ga, giga-annum; Ma, mega-annum; Sr, strontium; after Karlstrom and Humphreys, 1998; Eisele and Isachsen, 2001; Foster and others, 2006). 4 Control of Precambrian Basement Deformation Zones on Emplacement of the Laramide Boulder Batholith, Montana

115° 105° TRANS- EXPLANATION 50° HUDSON HEARNE MEDICINE HAT Vulcan structure Cretaceous batholithic rocks CANADA UNITED STATES Tectonic boundary of Precambrian basement terrane—Dashed where inferred

PRIEST  Thrust-faulted tectonic boundary of Precambrian basement terrane—Dashed where inferred RIVER 

Approximate tectonic boundary between unexposed Archean (to southeast)  GFTZ and Proterozoic (to northwest) basement terranes in Great Falls tectonic zone (GFTZ)

Approximate boundary between 87Sr/86Sr<0.706 in intrusive rocks to BB  the west and 87Sr/86Sr>0.0706 to the east 4 3 5 WYOMING Approximate locations of samples and upper intercept uranium-lead 2  2 WA 1 TR zircon dates in millions of years before the present (Ma) 1. 1,760 Ma and 2,647 Ma  MT 45° PB Area of figure 3 2. 2,720 Ma WY 3. 2,450 Ma SELWAY  IB ID 4. 1,700 Ma 5. 2,470 Ma 

0 200 KILOMETERS

0 200 MILES

Figure 2. Upper intercept zircon dates from intrusive rocks in the Great Falls tectonic zone (GFTZ). The terranes and boundaries are as shown for the figure 2 reference area in figure 1 (BB, Boulder batholith; IB, Idaho batholith; PB, Pioneer batholith; TR, Tobacco Root batholith; data from Lund and others, 2002; Murphy and others, 2002).

The Boulder Batholith axes and concurrent reverse faulting, indicating that folding, faulting, and intrusion were synchronous in many instances. This finding is consistent with observed thrust-faulted, sill/ Plutonism and cogenetic volcanism in the Boulder batho- host-rock contacts that postdate sill emplacement (Smedes, lith began approximately 81 Ma (Tysdal, 1998) and continued 1966). Paleomagnetic studies show that some sills were folded until approximately 74 Ma (Lund and others, 2002). The after emplacement while others intruded into folded rocks (S. batholith (fig. 7), elongated northeast-southwest, is approxi- Harlan, George Mason University, personal commun., 2005). mately 90 km long and 50 km wide. Based on differing styles Many early-stage plutons cut and offset thrust faults and of structural control on pluton emplacement, the plutonic related sill complexes and were clearly emplaced after local rocks are subdivided into two groups, an early stage and a younger Butte Quartz Monzonite stage. Early-stage plutons thrust faulting (fig. 8). Many of these plutons and related span the approximate interval of 81–76 Ma (Lund and others, hydrothermal veins strike northwest parallel to high-angle 2002), while the more voluminous Butte Quartz Monzo- faults. At some localities, screens of Elkhorn Mountains nite was emplaced over a shorter interval at approximately Volcanics have a strong northwest-trending shear fabric, and 75–74 Ma (Lund and others, 2002). adjacent northwest-southeast–elongated intrusions have a distinct contact-zone foliation, both abruptly terminated at a sharp, northeast-striking contact with the younger stage Butte Tectonic and Structural Localization of Early- Quartz Monzonite. These data are suggestive that the plutons Stage Magmas were localized by northwest-striking faults. The localization of early-stage intrusions by post-thrust Initially, early-stage magmas were emplaced concurrently faulting, northwest-striking faults characterizes much of the with superjacent thrust faulting and related folding. The oldest east and south margins of the Boulder batholith (see Klepper cogenetic Elkhorn Mountains Volcanics are folded and uncon- and others, 1957; Prostka, 1966; Smedes, 1966; Tilling, 1968; formably overlain by younger Elkhorn Mountains Volcanics Klepper and others, 1971b). Figure 8 shows the relation of that are not folded (Smedes, 1966). Thrust faults occurring early-stage intrusions and associated hydrothermal veins to after sill emplacement commonly juxtapose Elkhorn Moun- northwest-striking faults along the east margin of the batho- tains Volcanics and sills. Many sills can be traced back into lith. The Silver Wave stock (SWS in fig. 8) is localized along plutons with lineation fabrics (Smedes, 1966). The thrust faults the left-lateral Indian Creek fault zone. The Indian Creek zone propagated folds in the upper and lower plates, and Klepper offsets reverse faults and early-stage sills. At the south end of and others (1957) documented north-trending synvolcanic fold the batholith, the Donald and Hell Canyon plutons (fig. 7) are The Boulder Batholith 5

112°00’ 10 5 5 5 5 10 5 5 5 40 10 5 10 50 5 15 20 25 5 25 5 15 45 45 10 5 10 5 20 5 30 10 5 50 40 5 35 20 5 0 60 10 20 20 30 10 50 30 10 15 35 60 30 30 20 60 35 10 20 5 50 20 45 20 10 40 5 60 50 30 60 30 20 20 40 60 50 20 45 50 70 10 20 30 45°17’30” 5 50 50 20 40 20 50 70 50 5 45 30 30 60 20 50 50 40 30 20 50 50 5 50 30

EXPLANATION 0 0.5 1 KILOMETER

Cenozoic volcanic and surficial deposits, undifferentiated 0 0.5 1 MILE Cenozoic 45 Strike and dip of foliation Pegmatite dike, Mesoproterozoic(?) 20 Lineation showing plunge

Proterozoic Zone of shear Quartzofeldspathic gneiss, undifferentiated Fault Amphibolite—Dotted where in subcrop Orogenic quartz vein, auriferous, dotted where in subcrop Archean Dolomitic marble—Dotted where in subcrop

Ultramafic rocks, undifferentiated

Figure 3. Lineations, foliations, and orogenic gold veins in Archean rocks in the southernmost Tobacco Root Mountains (from Weir, 1982). The location of this figure is shown on figure 2, and the location of the Tobacco Root Mountains is shown by the symbol TRM on figure 5. Syntectonic, reverse-fault–controlled auriferous quartz-sulfide-carbonate-fuchsite veins are indicated by red lines. The abundant west-northwest–east- southeast trending pegmatite dikes are 1,572±51 Ma (mega-annum) (Cole, 1983) and represent an imposed Mesoproterozoic structural fabric. 6 Control of Precambrian Basement Deformation Zones on Emplacement of the Laramide Boulder Batholith, Montana

115° 105°

TRANS- 50° HUDSON EXPLANATION HEARNE MEDICINE HAT Vulcan structure Proterozoic Belt Basin CANADA UNITED STATES Tectonic boundary of Precambrian basement terrane—Dashed where inferred PRIEST RIVER   Thrust-faulted tectonic boundary of Precambrian basement terrane—Dashed where inferred GFTZ Approximate tectonic boundary between unexposed Archean (to southeast) and Proterozoic (to northwest) basement terranes in Great Falls tectonic zone (GFTZ) GL  Approximate boundary between 87Sr/86Sr<0.706 in intrusive rocks to the west 87 86  and Sr/ Sr>0.0706 to the east WA PL Interpreted axes of Proterozoic rift basins Proterozoic Helena embayment

 45° MT WY En echelon zone of faults delimiting northeast margin of Lewis and  Clark line, predominantly strike-slip ID WYOMING SELWAY 

0 200 KILOMETERS

0 200 MILES

Figure 4. Outline of the Belt Basin on a part of the basement terrane map shown in figure 1. The figure also shows the orientations of normal faults that were active across the basin during its development, the northeast margin of the Lewis and Clark line, the margins of the Great Falls tectonic zone (GFTZ), the projection of the main suture between the Archean Wyoming block to the southeast and the Proterozoic Medicine Hat block to the northwest, and the location of the northern and southern structural margins of the Helena embayment. These structural zones are known by various names including the Garnet line (GL) in the north and the Perry line (PL) in the south.

112°30’ 112°00’

    EXPLANATION        HM  Undifferentiated Phanerozoic rocks  

 Cretaceous Boulder batholith    HC  Cretaceous Tobacco Root batholith

 

  Paleozoic sedimentary rocks  

45°30’  Meso- and Neoproterozoic dikes TRM Mesoproterozoic Belt Supergroup rocks 

Archean crystalline rocks  



 Strike-slip faults, sense of offset shown where known   Thrust faults, teeth on upper plate

Normal faults, bar and bell on downthrown wall



 





   45°00’ 0 5 10 KILOMETERS

0 5 10 MILES

Figure 5. Generalized geologic map of a part of the Highland Mountains (HM) at the southern end of the Boulder batholith. The Hell Canyon pluton (HC) and Tobacco Root Mountains (TRM) are shown for reference. Mesoproterozoic and Neoproterozoic dikes are shown in black. As mapped, the northwest-striking left-lateral faults are a Mesozoic reactivation of originally Mesoproterozoic normal faults (after Harlan and others, 1996). The Boulder Batholith 7

110° 118° 114°

50°

B

A

R

L

I

T

B

I

E

S

R H

T

C A

O

L

U M 

B  I

A   M

CANADA O

 N  T UNITED STATES A N A

 

 D   I  ST  

 U

 Libby  R B

 E  D

 

 Trough 

B

Coeur E 

L 

T d’Alene 





L   EW  Helena

I  S structural 

   salient  

 AN  D

47°   

C   

LA  R   K   L

 I  N 

 E 



  BB   Butte WA IB 

ID  OR

Willow Creek fault



MT

 

 

 WY



 

0 100 MILES 

 

 ID

0 100 KILOMETERS 44°

EXPLANATION

Approximate area of Cretaceous batholithic and related volcanic rocks

Approximate limit of outcrop of Proterozoic Belt Supergroup rocks

Strike-slip faults, sense of offset shown where known

 Thrust faults, teeth on upper plate

Normal faults, bar and bell on downthrown wall when known

Figure 6. Generalized tectonic map of southwestern Montana during the Late Cretaceous showing the principal thrust-fault complexes, regional-scale faults, and locations of the Proterozic Belt Basin and Cretaceous batholithic complexes. The Cretaceous Montana disturbed belt corresponds with the northeast margin of the Belt Basin, and the Helena structural salient approximately corresponds to the Helena embayment of the Belt Basin. The region of deformation known as the Lewis and Clark line is shown for reference (BB, Boulder batholith; IB, Idaho batholith; after Harrison, 1972). 8 Control of Precambrian Basement Deformation Zones on Emplacement of the Laramide Boulder Batholith, Montana

112°45' 112°30' 112°15' 112°00' 111°45' 111°30'

Area of figures 10 & 12 HELENA

Elliston KR Unionville TOWNSEND 46°30' U Rimini VALLEY

Clancy

Y E

L Area of

L Emery

A Corbin figure 8

V

E Wickes G D Comet O

L

R

E 46°15' E Basin Elkhorn D

Boulder

46°00' Butte

Data for figure 11 from this area

CG D EXPLANATION BP RC Alluvial deposits, undivided Fault, sense of displacement

Holocene undivided

D Pliocene to Mine MC Lowland Creek Volcanics Early Principal cities Eocene Boulder batholith Small communities associated HC Butte Quartz Monzonite with mining districts Diorite to granodiorite, undivided Basin Mining district name

Late Gabbro HC Pluton name, abbreviated Cretaceous 0 10 KILOMETERS Elkhorn Mountains Volcanics Pre-Elkhorn Mountains Volcanics, undivided; includes 0 10 MILES Mesozoic, Paleozoic, and Precambrian formations

Figure 7. Generalized geologic map of the Boulder batholith. Individual plutons labeled are Burton Park (BP), Climax Gulch (CG), Donald (D), Hell Canyon (HC), Kokoruda Ranch Complex (KR), Moose Creek (MC), Rader Creek Granodiorite (RC), and Unionville Granodiorite (U). Black circles denote locations of major cities in the area, black squares denote locations of selected mining districts, and the general distribution of mines is indicated by the black and white shaft symbols (after Klepper and others, 1957; Becraft and others, 1963; Knopf, 1963; Ruppel, 1963; Prostka, 1966; Smedes, 1966, 1967, 1968; Klepper and others, 1971b). The Boulder Batholith 9

111°40’

U D

SWS

T

L 46°20’ U A

F

L

L

I

H

D N

SWS O

M

A

I

D

U D

I ND IAN C RE EK F AU LT

0 0.5 1 MILE

0 0.5 1 KILOMETER EXPLANATION Alluvial deposits, undivided U Reverse fault—Dashed where inferred, dotted where D

Pliocene buried; arrow indicates dip direction; Holocene to U on upthrown side of fault, D on downthrown side Diorite to granodiorite, undivided Anticlinal axis—Dotted where buried

Late Elkhorn Mountains Volcanics Synclinal axis—Dotted where buried Cretaceous Mineralized quartz veins Pre-Elkhorn Mountains Volcanics rocks, undivided Fault, undivided—Dashed where inferred, dotted where buried Strike-slip fault—Dashed where inferred, dotted where buried; arrows indicate sense of slip

Figure 8. Generalized geologic map of a part of the east margin of the Boulder batholith showing the relation of (1) early-stage sills to north-striking reverse faults and intrusions and (2) related quartz-sulfide veins to northwest-striking strike-slip faults (SWS, Silver Wave stock; after Klepper and others, 1971b). 10 Control of Precambrian Basement Deformation Zones on Emplacement of the Laramide Boulder Batholith, Montana elongated northwest-southeast, and, in the Hell Canyon stock, (Klepper and others, 1971a). Also along the east boundary, Tilling (1968) mapped discrete compositional zones whose Schmidt and others (1990) report that mafic enclaves are long axes parallel the margins of the stock. Thus, field rela- abundant and the enclaves are flattened parallel to the pluton tions are indicative of a temporal evolution during early-stage margin. Adjacent to the margin, there is commonly also a magmatism from west-dipping reverse faulting to strike-slip foliation fabric in the pluton parallel to the enclaves. Schmidt accommodation of shear in the vicinity of the batholith. and O’Neill (1982) and Schmidt and others (1990) interpret The Precambrian heritage of the northwest-striking faults the above fabrics as thrust-fault related and evidence that the that localized early stage plutons is circumstantial, but the east margin of the Butte Quartz Monzonite is structural and spatial correlation of the Hell Canyon pluton (fig. 7) and Pre- follows the trace of a pre–Butte Quartz Monzonite thrust fault. cambrian faults and dikes (fig. 5) is permissive and supported by geophysical data. Linear trends in the regional gravity and Late-Stage Dikes and Veins magnetic data defined after removal of those correlating with topographic trends strike almost exclusively northwest (fig. 9). With the exception of the earliest magmas, the Boulder Some of these may be traced laterally into mapped Protero- batholith plutons postdate thrust faulting; farther east and zoic faults in exposed basement rocks. Therefore, we assume north, the frontal thrust zone was active into the Paleocene that most northwest-trending basement lineaments through- (Mehnert and Schmidt, 1971; Harlan and others, 1988). Thus, out the mapped area are reflective of Proterozoic basement for the duration of batholith emplacement, the entire region deformation. was under compression with the maximum principal stress There are a number of sill complexes along the west horizontal and oriented east-west, a condition that persisted margin of the batholith that have been interpreted previously through the Paleocene. as indicating that the west margin of the batholith is lacco- The final stage of the Butte Quartz Monzonite magma- lithic (for example, Lawson, 1914; Schmidt and others, 1990; tism consists of the emplacement of numerous pegmatite, O’Neill and others, 2004). As discussed below, the potential alaskite, and aplite dikes (fig. 10). Detailed mapping of dikes field data do not support the laccolith hypothesis, and we (Becraft and others, 1963; Ruppel, 1963; Smedes, 1966; interpret the sills on the west side of the batholith as analogous Houston, 2002) provides insights regarding the state of stress to synthrust fault-related sills on the east and north margins of that controlled fluid flow during the final emplacement of the batholith. Butte Quartz Monzonite magmas. Figures 10 and 11 show rose diagrams of dike orientations of Butte Quartz Monzonite Tectonic and Structural Localization of Later- dikes as well as early stage dikes in the northeast part of the Stage Magmas batholith. Within the Butte Quartz Monzonite, a preferred northeast orientation contrasts with the more equally distrib- uted northeast and northwest orientations exhibited by dikes Although the later-stage intrusions are texturally hetero- associated with pre–Butte Quartz Monzonite, Boulder batho- geneous, they are sufficiently homogeneous in composition to lith intrusions. Most of the analyzed dikes crop out within render their contacts generally indistinguishable in the field. a 4- to 6-km-wide northeast-trending corridor that extends As a consequence, excepting the Climax Gulch pluton (fig. 7), obliquely across the entirety of the pluton. Near Butte, in the the later-stage intrusions are mapped as a single unit, the Butte southern part of the batholith and in contrast to the northern Quartz Monzonite. In contrast to the irregular geometries part of the batholith, there is a strong, generally east-west pat- of the early stage plutons, the Butte Quartz Monzonite is a tern evident in the dike orientations (fig. 11). remarkably symmetric mass with parallel north and south as Immediately following the crystallization of the Butte well as east and west margins. The long, northeast-trending Quartz Monzonite and related dikes, sulfide-bearing quartz axis of the pluton is 90 km long, and it is about 50 km wide. veins were formed in the northern half of the Butte Quartz The Butte Quartz Monzonite generally has a flat contact with Monzonite (fig. 12) with a predominant east-west strike. its roof rocks (Hamilton and Myers, 1967). Younger rocks mask the western margin of the Butte Quartz Monzonite, but the other boundaries are sufficiently Geophysical Model of the Butte Quartz well exposed to provide information regarding structural controls along the margins. Along the southeast margin of the Monzonite Butte pluton, Prostka (1966) mapped a contact-parallel and commonly vertical shear fabric in an early-stage diorite and To obtain a three-dimensional (3D) image of the Boulder the overlying Elkhorn Mountains Volcanics that is truncated batholith and provide a context for interpreting mechanical by the Butte Quartz Monzonite. Along the same contact farther controls on emplacement of the batholith, we synthesized southwest, there is a screen of pre-Mesozoic rocks between magnetic, gravity, petrological, geochemical, and seismic data the Butte Quartz Monzonite and the early-stage Rader Creek and applied modern interpretative techniques. Owing to the Granodiorite pluton (fig. 7). Some beds within these sedi- sharp density contrast between the lower density siliceous mentary rocks are vertical to overturned against the contact igneous rocks and their denser sedimentary and metamorphic Geophysical Model of the Butte Quartz Monzonite 11

113°00’ 112°30´ 112°00’

Garnet line

Elevation (kilometers) 46°30´

h lit 3.0 batho er ld u o B

f o

e g 2.6 d e

r e Out

2.2 46°00’ Butte

Perry line 1.8

1.4

45°30´ 1.0

0 50 KILOMETERS

0 50 MILES EXPLANATION Tectonic terrane boundary as interpreted Linear trend in aeromagnetic data from aeromagnetic and gavity data Linear trend in gravity data Linear trend of faults: Perry line and Garnet line Faults shown on published geologic maps

Figure 9. Topographic map (highest elevations in red, lowest in blue) showing linear trends in the regional gravity and magnetic data defined after removal of those correlating with topographic trends. The outer, solid white line boundary delimits the geophysical margin of the Boulder batholith; the inner, dashed white line delimits the outer geophysical margin of the Butte Quartz Monzonite phase of the batholith. The location of the city of Butte (star) shown for reference. 12 Control of Precambrian Basement Deformation Zones on Emplacement of the Laramide Boulder Batholith, Montana

112°30' 111°45' 0° B, C A 330° 30° 300° 60°

270° 90°

46°30' 240° 120° 210° 150° 180° n = 513 B 0° 330° 30° 300° 60°

270° 90°

240° 120° 210° 150° 180° n = 685 C 0° 330° 30° 300° 60°

46°15' 270° 90° Boulder 240° 120° 210° 150° 180° n = 42

0 5 10 KILOMETERS

0 5 10 MILES

EXPLANATION

Alluvial deposits, undivided Fault, undivided Pliocene

Holocene to Principal city Lowland Creek Volcanics Early Eocene Boulder batholith Late-stage dikes, predominantly granite pegmatite and alaskite Butte Quartz Monzonite Diorite to granodiorite, undivided Late

Cretaceous Gabbro

Elkhorn Mountains Volcanics

Pre-Elkhorn Mountains Volcanics, undivided; includes Mesozoic, Paleozoic, and Precambrian formations

Figure 10. The distribution and orientation of early- and late-stage aplite, pegmatite, and alaskite dikes on a generalized geologic map of the northern half of the Boulder batholith (after Becraft and others, 1963; Ruppel, 1963; Smedes, 1966). Inset rose diagrams A and B pertain to measured orientations of late-stage dikes in the Butte Quartz Monzonite pluton; A, measured outside of the outlined area; B, measured within the outlined area. Inset rose diagram C pertains to early-stage dikes measured within the outlined area. host rocks, gravity data were used to determine the shape Geophysical Data and Modeling of the batholith. The lateral extent of the batholith beneath volcanic and surficial cover was determined from the analysis A reduced-to-pole aeromagnetic map of the Boulder of magnetic data. Structural trends within and surrounding batholith and vicinity and a residual short-wavelength mag- the batholith were highlighted through the filtering of the netic anomaly map of the same region are shown in figure 13. potential-field data. The inversion of gravity and magnetic The magnetic data were extracted from a previously compiled data provided a base from which a 3D model and visualization digital dataset composed of several aeromagnetic surveys of the Butte Quartz Monzonite could be constructed. (McCafferty and others, 1998). Survey line spacing in the Geophysical Model of the Butte Quartz Monzonite 13

compensate for the load on the crust caused by the additional mass of mountains, the isostatic residual gravity anomaly was 0° determined (Simpson and others, 1986), which reflects more 330° 30° closely the contribution of intracrustal density variations. The complex magnetic and gravity patterns shown in 300° 60° figures 13 and 14 indicate anomalies from a variety of sources lying at different depths. This superposition of anomalies can result in interpretational ambiguities. To locate near-surface 270° 90° magnetic and gravity sources, regional anomalies were first removed to isolate short-wavelength anomalies of interest (the residual maps), and then horizontal-gradient analysis aided

240° 120° in locating the edges of these sources (the small circles on residual maps). The magnetic and gravity data were continued upward analytically 1 km to generate regional fields. These 210° 150° regional or low-frequency-passed fields were then subtracted 180° from the unfiltered dataset to derive the residual or high- n = 193 frequency-passed fields. Combining the unfiltered and residual magnetic data in this manner highlights subtle geologic infor- Figure 11. Rose diagram for late-stage dikes mation for different depths. Several linear geophysical features in the Butte Quartz Monzonite in the southern became apparent. half of the batholith. The predominance Two filtering approaches were used to enhance geophysi- of north-south and east-west orientations cal boundaries. The first method employs horizontal gradient contrasts with preferred dike orientations in analysis, which is an analytical mapping tool that defines the northern half of the Butte Quartz Monzonite interpreted rock-unit boundaries and their orientation on the as shown in figure 10. The map area from basis of local curvature of the magnetic potential field or grav- which the orientation data were made is shown ity field (Grauch and Cordell, 1987). Maxima in the horizontal in figure 7 (after Smedes, 1967, 1968; Houston, gradient of these fields occur near steep boundaries separat- 2002). ing contrasting magnetizations or densities. The locations of high horizontal gradients, automatically determined (Blakely and Simpson, 1986), are shown in figures 13B and 14B as the alignment of very small black circles. In the second method, a study area is 1.6 km or less, except in small areas in the shaded-relief filter was applied to the residual magnetic data to extreme northeast and southeast corners of the area where further enhance magnetization boundaries. the spacing is 3.2 km. A 1-km grid of magnetic values was Before applying a 3D modeling technique, a two-dimen- prepared for each aeromagnetic survey using a minimum sional (2D) modeling program based on Webring (1985) was curvature algorithm (Webring, 1981). All data were analyti- used to understand the lateral variations of total magnetization cally continued to a surface 305 meters (m) above the terrain. and density along ten selected profiles shown in figures A13 Assuming induced magnetization or remanent magnetization and 14A, respectively. In the modeling process, the bodies aligned or nearly aligned in the direction of Earth’s magnetic are assumed to extend a specified distance normal to the field, the magnetic-anomaly data were reduced to the magnetic profile. The program uses nonlinear inversion and requires an pole, a procedure that shifts magnetic anomalies to positions initial estimate of model parameters including depth, shape, above their sources. density, and total magnetization of suspected terranes. The An isostatic gravity anomaly map of the Boulder batho- program adjusts the values of selected parameters such that lith and vicinity and a residual short-wavelength gravity the weighted root mean square errors are minimized between anomaly map of the same region are shown in figure 14. The the observed and calculated magnetic and gravity fields. The gravity data were taken from the National Geophysical Data interpretation of either magnetic or gravity data yields non- Center database (approx. 4,000 stations). All data were tied to unique solutions because a large number of geometric models the IGSN-71 gravity datum (International Gravity Standard- will have an associated field that closely matches the measured ization Net 1971) and reduced to complete terrain-corrected field. In the derivation of a suitable geophysical model to rep- Bouguer-anomaly values using a reduction density of 2,670 resent the geologic setting of the Boulder batholith, drill-hole kg/m3 (kilogram per cubic meter) and the 1967 formula information, physical property data, seismic interpretations, for the theoretical gravity of the Earth (Cordell and others, and sound geologic reasoning were used. 1982). The spacing of gravity stations is generally adequate To translate observed magnetic and gravity anomalies (about 1 station per 5.5 km2) to define and characterize upper into a 3D geologic picture of the subsurface, we used an crustal features within the study area. A 1-km grid of gravity inverse modeling computer algorithm (Cordell and Hender- values was prepared using the Webring (1981) algorithm. To son, 1968) that solves for the thickness of a density layer with 14 Control of Precambrian Basement Deformation Zones on Emplacement of the Laramide Boulder Batholith, Montana

112°30' 111°45' 0° B A 330° 30° Elliston 300° 60°

Unionville 270° 90° 46°30' 240° 120° Rimini 210° 150° Clancy 180° n = 132

0° B 330° 30° 300° 60°

270° 90° Corbin 240° 120° 210° 150° Wickes 180° Comet n = 193

Basin Elkhorn 46°15' Boulder

0 5 10 KILOMETERS

0 5 10 MILES

EXPLANATION

Alluvial deposits, undivided Fault, undivided

Pliocene Principal city Holocene to

Lowland Creek Volcanics Communities associated with

Early mining districts Eocene Boulder batholith Mine Late-stage polymetallic veins

Butte Quartz Monzonite

Diorite to granodiorite, undivided Late

Cretaceous Gabbro

Elkhorn Mountains Volcanics

Pre-Elkhorn Mountains Volcanics, undivided; includes Mesozoic, Paleozoic, and Precambrian formations

Figure 12. The distribution and orientation of latest-stage sulfide-bearing quartz veins on a generalized geologic map of the northern half of the Boulder batholith. A, Inset rose diagram pertaining to orientations measured outside of the outlined area; B, Inset rose diagram pertaining to orientations measured within the outlined area (after Becraft and others, 1963; Ruppel, 1963; Smedes, 1966). Geophysical Model of the Butte Quartz Monzonite 15 0 50 150 100 –50 –100 –150 –200 nT , Reduced-to-pole A 112°00’ Garnet line Perry line 112°30’ 50 MILES Short-wavelength magnetic anomalies 113°00’ , Aeromagnetic map based on the shorter wavelength portion of B 50 KILOMETERS B 25 0 360 240 120 –120 –240 –360 nT 25 2 9 0 0 8 112°00’ 10 7 112°30’ Magnetic field 3 6 Reduced-to-pole aeromagnetic maps of the Boulder batholith and vicinity. The outer bold white line is the interpreted geophysical margin Reduced-to-pole aeromagnetic maps of the Boulder batholith and vicinity. 1

2 113°00’ 5 4 A Figure 13. nanotesla) of the batholith. The inner bold white line is interpreted outer geophysical margin Butte Quartz Monzonite. ( nT, aeromagnetic map. The numbered black lines show the locations of profiles used to model batholith. Solid (1, 3, 5, 8) indicate those shown in figure 16. The location of the Butte mining district is by a star symbol. spectrum to emphasize shallow features. The small dots are inflection points in the magnetic wave forms. Lines of ar e interpreted as boundaries between rocks with differing physical properties. The bold white dashed lines indicate the locations of Garnet line and Per ry line, shown for reference. 46°30’ 46°00’ 45°30’ 16 Control of Precambrian Basement Deformation Zones on Emplacement of the Laramide Boulder Batholith, Montana 0 3.0 2.0 1.0 –1.0 –2.0 –3.0 –4.0 mG , Isostatic gravity A 112°00’ 112°30’ 50 MILES 113°00’ 50 KILOMETERS B 8 0 32 24 16 –8 –24 25 –16 mG 25 0 0 , Residual isostatic gravity anomaly map. The small dots are inflection points in the B 112°00’ 112°30’ Gravity maps of the Boulder batholith and vicinity. The outer bold white line is the interpreted geophysical margin of Gravity maps of the Boulder batholith and vicinity.

113°00’ A Figure 14. batholith. The inner bold white line is the interpreted outer geophysical margin of Butte Quartz Monzonite. (mG, milligal) anomaly map. The numbered dashed black lines show the locations of profiles used to model batholith. Larger numbers (1, 3, 5 , 8) indicate (LT) and southwest Montana those profiles shown in figure 16. The bold white dashed lines are major tectonic lineaments: Lombard thrust Mineral Resource Data Base are shown with pink-filled tectonic zone (SWMTZ). Mining localities documented in the U.S. Geological Survey’s AR, circles. Black dots are data collection sites. The interpretation of sources for the major anomalies abbreviated as follows : A, Avon; Proterozoic PB, Pioneer batholith; P, FL, Fleecer Stock; GC, Clark Fork River valley; DL, Deer Lodge Valley; Archean rocks; BH, Big Hole Valley; Root batholith. Belt Basin rocks; and TB, Tobacco magnetic wave forms. Lines of small dots are interpreted as boundaries between rocks differing physical properties. 46°30’ 46°00’ 45°30’ Geophysical Model of the Butte Quartz Monzonite 17 a specified flat upper surface, herein selected as the mean the margins of the batholith. The top layer is less than 1.3 km topographic elevation. The gravity field in the vicinity of the deep and is probably related primarily to the noise and errors Boulder batholith and a density contrast of 150 kg/m3 between within the datasets. Outside the boundary of the batholith, the batholith and surrounding country rocks were used. the second layer is at depths of 3.3 km and 2.8 km below the Because rock density varies along the flanks of the batholith, ground surface based on the spectrum of magnetic and gravity as suggested in the 2D models, the assumption of uniform data, respectively. These depths are likely related to a combi- density contrasts may locally be much greater than 150 kg/m3. nation of sources lying roughly, for example, at the average Thus, the calculated values in the 3D model are moderately depth of pre-Mesoproterozoic metamorphic rocks or the base unconstrained and only the relative changes in the thickness of volcanic fields. Within the batholith, the calculated depths should be considered and interpreted as major changes in the of the third layer from the magnetic and gravity data are bulk mass properties of the batholith. 5.9 km and 5.4 km below the surface based on the spectrum of magnetic and gravity data, respectively. We interpret the resulting average of 5.7 km to approximate the thickness of Results of Geophysical Data Analysis the batholith. Figures 13A and 14A show the locations of the profiles The magnetic data (fig. 13) primarily reflect isolated plu- used in the 2D modeling. Model results for profiles 1 and 5 are tons or clusters of plutons, as well as magnetization variations shown in figure 15 and for profiles 3 and 8 in figure 16. In the in Cretaceous and Eocene volcanic fields and the plutonic initial model, the bottoms of the upper crustal layer and of the rocks (fig. 7). The outer boundary of the Boulder batholith is batholith defined in the spectral analysis were assumed to be clearly defined by a magnetic boundary (fig. A13 ) separating 1.2 km and 4 km below sea level, respectively. However, this pronounced positive anomalies over the more mafic plutons assumption did not produce acceptable results for the assumed of the batholith from lower intensity anomalies lying outside densities of basin fill (2,200 kg/m3), Butte Quartz Monzonite the batholith. The magnetic highs associated with the early- (2,670 kg/m3), and the middle crustal layer (2,820 kg/m3). stage plutons form a remarkable, symmetrical ring around the Therefore, along most profiles, it was necessary to increase the late-stage Butte Quartz Monzonite. Magnetic expression of depth of the bottom of the upper crustal layer to about 3 km the ring is absent only in a short segment along the southwest below sea level and of the batholith to about 6 km below sea edge of the batholith. The inner edge of the ring (fig. 13A) is level. Thus, for the assumed densities, the batholith thickness the Butte Quartz Monzonite. The Butte Quartz Monzonite is is approximately 7.8 km. reflected in much lower intensities in the magnetic (fig. 13) Profiles 1 (fig. 15A) and 3 (fig. 16A) cross the entire and gravity (fig. 14) fields. About halfway from the northern to batholith, while profiles 5 (fig. 15B) and 8 (fig. 16B) cross only southern boundaries of the Butte Quartz Monzonite, the outer an edge of the batholith. The calculated fields of the simple higher intensity magnetic ring correlative with the early-stage sources that are shown reasonably match the observed fields. plutons appears on both sides of the batholith to indent the Analogous to its outcrop shape, the batholith is also a remark- center of the batholith (fig. 13A). This geometry is suggestive ably uniform rhombohedral shape in 3D. The outer contact of that there may be two separate lobes of the Butte Quartz Mon- the batholith is nearly vertical in all studied profiles, whereas zonite that coalesced across the top of underlying early-stage the boundaries between the early-stage, moderately to strongly intrusions not wholly evident in the potential-field data. magnetic (1.1–2.3 A/m [ampere-turn per meter]), and dense The gravity data delineate the major rock types in the (2,680–2,770 kg/m3) plutons and the Butte Quartz Monzonite study area. Primarily low-density basin fill, plutons, and vol- (1 A/m and 2,670 kg/m3) dip inward from 15° to 82° and, most canic rocks form prominent gravity lows (fig. 14A) in contrast commonly, between 45° and 60°. Along profile 3 (fig. 15B), to denser Paleozoic and Precambrian rocks. Low-density vol- the strong magnetic source (2.3 A/m) in the outer ring at the canic and plutonic rocks associated with the Boulder batholith north end of the batholith corresponds with an east-west– are delineated by a pronounced northeast-trending negative elongated pluton known as the Unionville Granodiorite gravity anomaly. Similar gravity lows in the southwest (PB (2.3–5.2 A/m) (fig. 7). in fig. 14A) and southeast (TB in fig. 14A) corners of the map The topography of the basal surface of the Butte Quartz are the edges of Late Cretaceous batholiths. Prominent gravity Monzonite derived in 3D modeling from a grid of vertical highs generally occur over dense plutons and Archean and rectangular prisms is depicted in figure 17. The walls of the Paleoproterozoic rocks such as mafic gneiss, amphibolite, and pluton dip inward to a generally flat bottom at 6- to 8-km iron formation, particularly south of the southwest Montana depth. Towards the north-central and south-central ends of tectonic zone (SWMTZ in fig. 14A). Less intense gravity highs the batholith, there are two zones with calculated depths of overlie moderately dense Paleozoic and Mesoproterozoic 13–15 km. This overall geometry suggests that the Butte sedimentary rocks. Quartz Monzonite magma was fed from depth principally Spectral inversion of magnetic and gravity data within a through two separate zones at the base of the batholith. Thus, 120-km2 area centered on the batholith required a three-layer overall, the plan outline of the Butte Quartz Monzonite (fig. 7) model to properly fit or model the radial amplitude spectra of is a composite of two funnel-shaped masses. The boundary of both magnetic and isostatic gravity data within and external to the two zones is interpreted to be where there are re-entrants 18 Control of Precambrian Basement Deformation Zones on Emplacement of the Laramide Boulder Batholith, Montana 45 SE 35 ρ =2,790; M=1.8 ρ =2,670; M=1.3 ρ =2,670; M=1.0 , A Boulder batholith 25 Distance (km) M=1.4 ρ =2,720 15 Upper crustal Phanerozoic sedimentary and volcanic rocks and Proterozoic Belt Supergroup Pre-Belt Supergroup Precambrian metamorphic rocks ρ =2,200; M=0 M=0.5 ρ =2,820 ρ =2,740; M=1.6 B 5 NW 0 2 0 2 4 6 8 -20 -30 -40 300 200 100 , Profile 5 is a section across the southwestern edge of Profile 5 -100 -200 -300 B 120 SE EXPLANATION 110 Boulder batholith, inner zone (Butte Quartz Monzonite) Boulder batholith, outer zone (granodiorite, diorite, gabbro) 100 ρ =2,820; M=0.5 ρ =2,850; M=2.3 90 ρ =2,790; M=0.5 80 M=1.2 70 ρ =2,700 Profile 3 Distance (km) 60 Locally dense or magnetic rocks (for example, plutons) Basin fill Boulder batholith M=0.5 ρ =2,670 50 40 , rock density; M, magnetic susceptibility; nT, nanotesla; mG, milligal; km, kilometer). , rock density; M, magnetic susceptibility; nT, 30 ρ Simultaneous inversion of magnetic and gravity data across the Boulder batholith showing computed actual gravi ty

ρ =2,770; M=0.7 ρ =2,770; M=1.4 A 20 NW

Profile 1 5 0 4 8

-5

15

-15 -25 (km)

500 300 100

-100 -300 -500 (mG)

Magnetics (nT) Magnetics

Depth Gravity Gravity Figure 15. fields (upper diagrams) and 2D model (lower diagram) for profiles 1 5. Locations of modeled are shown in figures 13A 14A. Profile 1 is a northwest-southeast cross section across the northern part of batholith. the batholith. ( Geophysical Model of the Butte Quartz Monzonite 19 NE 35 M=0.2 ρ =2,750 30 ρ =2,200; M=0 ρ =2,820; M=0.5 25 M=0.3 ρ =2,790 , A 20 M=1.5 ρ =2,770 Distance (km) 15 10 M=1.5 ρ =2,670 , Profile 8 is a section across the Boulder batholith B Upper crustal Phanerozoic sedimentary and volcanic rocks and Proterozoic Belt Supergroup Pre-Belt Supergroup Precambrian metamorphic rocks 5 M=1.0 ρ =2,670 B SW 0 Profile 8 0 0 2 4 2 6 8 0 10 -10 -20 -30 100 -200 -400 -600 NE 135 M=0.5 ρ =2,750 EXPLANATION 125 M=2.3 115 ρ =2,730 Boulder batholith, inner zone (Butte Quartz Monzonite) Boulder batholith, outer zone (granodiorite, diorite, gabbro) 105 95 Profile 1 85 75 Boulder batholith ρ =2,670; M=1.0 Distance (km) 65 , rock density; M, magnetic susceptibility; nT, nanotesla; mG, milligal; km, kilometer). , rock density; M, magnetic susceptibility; nT, ρ 55 Basin fill Locally dense or magnetic rocks (for example, plutons) ρ =2,680; M=1.1 45 35 M=1.1 ρ =2,740 25 Simultaneous inversion of magnetic and gravity data across the Boulder batholith showing computed actual gravi ty

A M=0.5 ρ =2,820 15 SW

Profile 3 8 4 0 0 4

-4

-14 -24 (km)

600 400 200

-200 -400 -600 -800 (mG)

Depth Magnetics (nT) Magnetics Gravity Gravity Figure 16. fields (upper diagrams) and 2D model (lower diagram) for profiles 3 8. Locations of modeled are shown in Figures 13A 14A. Profile 3 is a southwest-northeast profile through the Butte mining district across center of batholith. northeastern edge of the batholith. ( 20 Control of Precambrian Basement Deformation Zones on Emplacement of the Laramide Boulder Batholith, Montana

112°40´ 112°00’ Depth 46°40´ (kilometers)

N 14.4

13.2

Feeder? 12.0

10.8

9.6 Denser granodiorite 8.4 at depth? 7.2

6.0

Deer Lodge Valley 4.8

3.6 46°00’ Butte 2.4 Feeder? 1.2

0.0

-1.2

-2.4

0 10 20 KILOMETERS

0 10 20 MILES

Figure 17. Contour map of calculated depths to the base of the Butte Quartz Monzonite using a 3D gravity inversion model. The average modeled depth of the base is 6 kilometers (km). The floor rapidly descends to depths exceeding 10 km in two areas interpreted to be feeder zones for the Butte Quartz Monzonite. The solid white line shows the geophysical outer boundary of the Boulder batholith; the inner dashed white line shows the geophysical outer boundary of the Butte Quartz Monzonite. The arrows indicate where the base of the batholith appears to pinch inward coincident with higher density batholithic rocks as indicated by magnetic data (see fig. 13A). The anomaly associated with the Deer Lodge Valley is a modeling edge effect. The Paleocene Butte Mining District 21 into the Butte Quartz Monzonite of more magnetic (fig. 13A) used to classify the vein systems (for example, Sales, 1914). and denser, early-stage intrusions. The rings of denser igneous Based on apparent offsets, east-west faults (referred to as material may be continuous at depth, but the available data are “Anaconda” system by Sales, 1914) were interpreted as inadequate to address this issue. earliest. They typically display normal-sense displacement. The total volume of the batholith is difficult to calculate. Northwest-striking faults (referred to as “Blue” system by As noted previously, different modeling assumptions and Sales, 1914) followed. Blue system faults display left-lateral approaches give different estimates of the thickness of the to left-lateral-oblique senses of displacement. Northeast to batholith. In studying 21 plutons with thicknesses geologically east-northeast faults (referred to as “Steward” system by known or determined from geophysical studies, McCaffrey Sales, 1914) were interpreted as youngest. Steward system and Petford (1997) show that plutons have a horizontal dimen- faults have a right-lateral sense of displacement. Weed (1912) sion (L) greater than their thickness (T), which conforms to a and Sales (1914) noted inconsistencies in this classification relationship based on a power law: scheme in that not all faults within a given system are exactly T = 0.12 L0.88. the same age, and faults in an interpreted younger system of Although erroneous estimates of thickness are anticipated in faults are sometimes offset by faults interpreted to be an older some situations using this equation, it leads to a reasonable system of faults. Despite such inconsistencies, this three-part value of 6.9 km for the Boulder batholith. Based on this result classification continues to be widely accepted (for example, and the different modeling results, we estimate that the base of Lund and others, 2002). the Boulder batholith is likely deeper than 5 km and shallower Based on several observations, we propose a different than 10 km. Assuming a thickness of 7 km, the volume of the interpretation of how the fault systems in the Butte mining batholith is approximately 35,000 km3. district formed and evolved. 1. As Sales (1914) noted, we observe that the northeast- to north-northeast–striking Steward faults are seldom The Paleocene Butte Mining District offset by mineralized faults of other orientations, and they are the most continuous along strike. A 10-million-year (m.y.) hiatus followed emplacement of the Butte Quartz Monzonite before intrusive activity began 2. Across the whole of the productive part of the dis- again at 66–65 Ma (Lund and others, 2002) in the vicinity of trict, with respect to the Steward faults, fault-veins Butte, Mont. East-west-striking and west-northwest-striking of the Anaconda system have a mean splay angle of Paleocene quartz porphyry dikes crop out in the heart of the 25°±10° (n=29), and Blue system veins have a mean Butte district and in the mountains to the east. Presumed splay angle of 71°±9° (n=66), indicating a great deal to reflect the presence of an east-west fracture zone in the of consistency in the fault systematics. district, the dikes were intruded on the eastern edge of the interpreted southern feeder zone for the Butte Quartz Monzo- 3. Each of the major Steward faults bends progressively nite. Paragenetically early porphyry-style copper-molybdenum clockwise, that is extensionally, along strike to form and later polymetallic-vein deposits in the Butte mining a broad arc. From north to south in the main part of district were formed 64–63 Ma (Snee and others, 1999; Lund the district, the axial trace of the arc strikes north- and others, 2002). The deposits are hosted in the Butte Quartz northeast to south-southwest (fig. 18). The Anaconda Monzonite above the inferred southern feeder zone. There are faults are most evident in the south-central part of the two styles of polymetallic vein mineralization. One contains district, and overall they are recognized where the predominantly silver, lead, zinc, and subordinate copper ores Steward faults have their most northeasterly strike. In with a gangue of quartz, pyrite, K-feldspar, calcite, sericite, contrast, Blue system faults occur along all reaches and chlorite. The second contains predominantly copper, with of the Steward faults irrespective of strike, and, to the subordinate zinc, silver, and lead, with a gangue of quartz, east of the axis of the arc, the northeast fault-veins sericite, and pyrophyllite. Since 1880, the district has pro- bend into the orientation of the Blue fault-veins. duced in excess of 9.2 megaton (Mt) of copper, 2.5 Mt of zinc, 4. There are distinctive zones of high-density horsetail 439 kiloton (kt) of lead, 679 megaounce (Moz) of silver, and normal faults between closely spaced east-northeast- 2.8 Moz of gold (Miller, 1973). to east-west–striking faults in the heart of the district. The Butte mining district is localized in a complex, north- At the eastern tips of the master faults, the horsetail east-striking system of right-lateral and related faults (fig. 18). faults form a clockwise swarm of splays that fan to The fault patterns shown in figure 18 reflect vein-controlling the right with mean splay angles to the master fault structures (see Sales, 1914; Meyer and others, 1968). Short- between 28°±7° and 35°±12° (n=40). dashed lines outline the locations of two porphyry-style deposits that underlie the vein systems. Traditionally, the 5. The faults and fault-veins parallel the directions intersections of different fault-vein orientations have been of joints in the Late Cretaceous Butte Quartz 22 Control of Precambrian Basement Deformation Zones on Emplacement of the Laramide Boulder Batholith, Montana

0 2,000 FEET

500 METERS LX

MN Continental deposit fault

S K L2

AN Continental

Berkeley deposit

EXPLANATION Copper-rich zone Fault, undivided Northeast-striking veins and faults Extensional stepover faults between the Berkeley East-west–striking veins and faults and Continental porphyry-style deposits Northwest-striking veins and faults Mine shaft Silver-rich zone L2 Mine shaft name Northeast-striking veins and faults General outline of porphyry-style deposit Northwest-striking veins and faults

Figure 18. Map of polymetallic quartz veins and faults in the Butte mining district. Those veins and associated faults that were predominantly mined for silver are color coded differently from those veins and faults predominantly mined for copper. Data for the dashed and solid lines are from veins on the 2,800-foot underground level, whereas data for the dashed lines in the southeast part of the map labeled as extensional stepover faults are from veins on the 4,600-foot underground level. (AN, Anaconda mine; K, Kelley mine; L2, Leonard No. 2 mine; LX, Lexington mine; MN, Mountain Consolidated mine; S, Steward mine; after Weed, 1912; Sales, 1914; Smedes, 1967, 1968; Meyer and others, 1968). Discussion 23

Monzonite. This correlation was noted originally by geophysical lineament data, we infer that some early batholith- Weed (1912), who observed mineralized replace- stage northwest-striking faults along the east and west sides ments of sheeted granite in the subsurface as well as of the batholith that localize dikes and some plutons (fig. 8) some outcropping sheeted joints filled intermittently were reactivated Proterozoic structures. More problematic is along strike with veins. the evidence from inside the Butte Quartz Monzonite. Here, structural trends as determined from fault and joint orienta- 6. The spacing of mineralized fractures is, in general, tions, dike orientations, and geophysical lineaments gener- irregular. ally strike northeast, but can they be linked to a Precambrian When taken as a whole, we suggest that these observations are precursor? The corridor of Late Cretaceous dikes through the consistent with an interpretation that faults in the district grew middle of the Butte Quartz Monzonite, the reactivation of this in a systematic and sequential manner wherein splay faults zone in the Paleocene at Butte, and again in the Eocene in the grew off the sides and especially at the tips of segmented, northern batholith (for example, Becraft and others, 1963), parallel northeast-striking right-lateral master faults. The together with the U-Pb zircon dates (fig. 2), which are strongly splay faults formed uniform sets of structures because they divergent on either side of a northeast-trending line that passes were guided by the characteristic orientation of joints and through Butte, provide strong evidence that a Precambrian fractures in the host Butte Quartz Monzonite. In this model, fault zone underlies the Butte Quartz Monzonite. Further, the northeast-striking faults are the master faults, and they samarium and neodymium isotope ratios in Eocene volca- appear to form a complex right-extensionally stepping array of nic rocks (Dudás and Ispolatov, 1997) near the city of Butte northeast-striking master faults within which numerous splay indicate a change in basement rock composition between areas faults were formed and propagated between their respective northeast and southwest of the district, consistent with the master structures. Our interpretation of the formation of the underlying fault hypothesis of O’Neill (1998). different fault sets (fig. 19) follows the models of Martel and Boger (1998) and Flodin and Aydin (2004). Although much of the strain was accommodated through Shape and Controls on Emplacement of the northeast-southwest extension parallel to the overall northeast Boulder Batholith trend of the vein-controlling master faults, curved traces of some splay faults between parallel master faults suggest that The thickness and controls on emplacement of the some measure of strain was accommodated through local Boulder batholith and their relation to other Late Cretaceous clockwise rotation of the stress field. The predominance of tectonic elements have been the subject of debate for many extension parallel to the direction of strike-slip displacement is years (for example, Lawson, 1914; Hamilton and Myers, particularly evident in the zones of fans of extensional horse- 1967; Klepper and others, 1974; Schmidt and others, 1990; tail faults (fig. 19). Kalakay and others, 2001). Klepper and others (1974) argued that the Boulder batholith is 10–20+ km thick, while Hamilton and Myers (1967) considered it to be a flat-bottomed sheet Discussion about 5 km thick. Schmidt and others (1990) modeled grav- ity data and calculated the thickness of the batholith to be about 15 km. They further proposed that the western half of Imprint of Proterozoic Deformation Fabrics on the batholith is laccolithic, whereas the eastern half is a thick Boulder Batholith prism. Kalakay and others (2001) interpreted the batholith to have been emplaced as a composite tabular body at 1–10 km. The Paleoproterozoic suturing and related events formed Our interpretation of the geological and geophysical strong northeast- and east-west–trending deformation fabrics data is consistent with some aspects of published models and including faults, foliations, and lineations. Mesoproterozoic in disagreement with others. In agreement with Hamilton and Neoproterozoic deformations formed prominent northwest and Myers (1967), our modeling shows the batholith to be a (fig. 4) and northeast as well as east-west fault fabrics (fig. 3). relatively thin slab, but our model disagrees with published The reactivation of these various fault systems during Late interpretations that the batholith thickness exceeds 10 km (for Cretaceous to Early Tertiary tectonism has been widely rec- example, Klepper and others, 1971a). Further, our modeling ognized (for example, Harrison, 1972; Schmidt and Garihan, of the physical property data does not support a laccolithic 1986. The influence of Precambrian faults on the emplacement geometry for the western half of the batholith, as proposed of the Late Cretaceous Boulder batholith is particularly notice- by Schmidt and others (1990). Based on geologic map and able in the correspondence of the north and south boundaries potential-field geophysical data, we believe the 80- to 76-Ma of the batholith with the north and south fault boundaries of early-stage intrusions, after an initial stage of synthrust fault the Mesoproterozoic Helena embayment (fig. 4). As noted emplacement, were localized primarily by strike-slip and above, some lineaments mapped geophysically (fig. 9) within linked faults that accommodated strains behind the eastward- and to the southeast of the batholith are co-linear with mapped migrating thrust fault front, as may be seen in dike orientations Proterozoic faults. Based on field data and the interpreted illustrated in figure 10C. 24 Control of Precambrian Basement Deformation Zones on Emplacement of the Laramide Boulder Batholith, Montana

A EXPLANATION

Pre-faulting joints

Northeast-striking, strike-slip faults; arrows indicate sense of slip East-west–striking wing faults spawned at the tips of actively propagating fault segments N Northwest-striking wing faults spawned at the tips of actively propagating fault segments Sites where porphyry-style mineralization formed prior to B final completion of fault meshes

Depiction in C of faults and fault veins in the Butte mining district, from figure 18; color coded to highlight hypothetical sets of fault meshes that formed through growth of new faults at fault segment tips as modeled in A and B; arrows indicate sense of displacement on northeast-striking strike-slip faults N

C N Discussion 25

The potential-field data indicate the later-stage Butte 3. 3D geophysical modeling (fig. 16) indicates that the Quartz Monzonite is a generally symmetric body with inward- Butte Quartz Monzonite magma was likely intruded dipping sidewalls, a generally flat top, and a generally flat through two feeder zones, one in the north and the bottom. Where the side-margin contacts are exposed, field data other in the south. This, taken together with the pos- are strongly suggestive that the northeast-elongate pluton was sible ringing of the north and south lobes by more fault bounded. The strike of the western and eastern margins magnetic early-stage intrusions is suggestive of a of the Butte Quartz Monzonite indicates a significant change binodal pull-apart structure. This structure explains in the dominant fault orientation from the more homogeneous the differences between the north and south Butte northwest-southeast and northeast-southwest orientations dur- Quartz Monzonite, such as the presence of abundant ing early-stage plutonism. Because all phases of the batholith Butte Quartz Monzonite-related quartz-sulfide veins were emplaced concurrent with thrust faulting, the major in the northern node but few in the south. change in strain accommodation that occurred at approxi- The mechanism by which the pull-apart opened is subject mately 76 Ma requires explanation. The structural model that best explains all of the available data and geometric relations to constraints set by field observations, seismic profiles, and is a pull-apart model similar to that proposed by Schmidt and modeling of the potential-field geophysical data. The geo- others (1990). metric relations that must be accounted for include the (1) Our modeling results and interpretation differ signifi- the geometry of the batholith, including the east dip of its cantly from the interpretation of Schmidt and others (1990): west edge and west dip of its east edge (see Vejmelek and Smithson, 1995); (2) bilateral symmetry of the Butte Quartz 1. The potential-field geophysical data show that the Monzonite across the zone of northeast-striking dikes, despite Butte Quartz Monzonite is ringed (fig. 13A) by mafic, the coeval asymmetric advance of the thrust front farther east more magnetic early-stage intrusions. (fig. 6); (3) its <10-km thickness; and (4) the flattening strains 2. In addition, the potential-field data preclude the in the east margin of the Butte Quartz Monzonite, along with west half of the Butte Quartz Monzonite from being a strong compression of the metamorphic rock or early-stage laccolithic, as discussed above. Owing to the oppos- pluton footwall of the contact. We suggest that the most likely ing displacement on east-west faults at the north and mechanism is a deep-seated, northeast-striking basement shear south sides of the Helena embayment as the thrust zone that extends diagonally beneath and across the long axis front continued to migrate east and the predominance of the Butte Quartz Monzonite, likely a reactivation of a large of northeast-directed shear coupled with east-west Paleoproterozoic suture, the Great Falls tectonic zone, as extension within the Butte Quartz Monzonite, the hypothesized by O’Neill (1998). The possibility of such a zone Butte Quartz Monzonite is best modeled as having was discussed previously, but Tertiary magmatism provides been localized within a pull-apart structure. additional corroborative evidence. Northeast-striking Eocene

Figure 19. (facing page) Structural model of the origin of the faults and fractures that localized veins in the Butte mining district. In A and B, pre-fault joint trends are shown in light gray; faults are color coded according to whether the predominant strike is northeast (blue), east-west (magenta), or northwest (green). A, The initial stage of faulting consists of along-strike segmented, parallel north- northeast–striking to northeast-striking right-lateral strike-slip faults developed parallel to northeast-striking joints. At along-strike segment boundaries, small splay or wing faults with trends of east-west and northwest may develop on one or both sides of the master faults at segment tips. Parallel splay faults that propagate away from a master fault on opposite sides of the master do not necessarily form a singular line and thus may be interpreted as having been a singular fault that was offset by the master. As the northeast faults continue to propagate along strike, new splay faults may develop and preexisting splays may lengthen primarily in the direction of a northeast-striking master fault and, in turn, spawn new splay faults at their propagating tips as the system of faults grows in complexity. B, With continued shear stress on the master northeast-striking faults, the splay faults progressively propagate away from the master faults on one or both sides of the individual masters and begin to form linked mesh-form arrays of faults. New faults in the orientation of the master fault may develop at tips of wing faults between regional masters and also begin to propagate, thereby resulting in the formation of subnetworks within the larger-scale networks of faults. At Butte, northwest-striking faults accommodate the most extension within the shear system and thus are the most continuous and abundant extensional splay faults. Such arrays of northwest faults may grow sufficiently to intersect another master fault and complete a set of “horsetailing” faults.C , The complex networks of linked faults continue to develop until the northwestern-most northeast master fault is kinematically linked to the southeastern-most master fault. The fault patterns at Butte shown here suggest that the system of linked faults consists of at least six major groups, interpreted and color coded individually for illustrative purposes. It is interpreted that the hypothesized fault network groups acted to compartmentalize hydrothermal fluid flow during vein formation, leading to the general segregation of silver-rich and copper-rich veins as shown in figure 18. 26 Control of Precambrian Basement Deformation Zones on Emplacement of the Laramide Boulder Batholith, Montana dikes in the northern node of the Butte Quartz Monzonite are epithermal and not mesothermal conditions, (2) plutons of all coincident with the Cretaceous dike trend (fig. 7) and form a ages in the batholith intrude their own volcanic pile, (3) the 3-km-wide, 50-km-long corridor. In addition, there are also roofs of the plutons are generally flat, (4) open-space filling northeast-striking Tertiary faults and Oligocene dikes and quartz-carbonate-sulfide veins presumed related to the batho- veins along the same corridor. These relations require the lithic plutons are found near the roof-country rock contact of batholith to be attached to the basement and not displaced east many stocks, and (5) some Elkhorn Volcanics eruptions, into on a décollement; further, they support the presence of a high- which the Butte Quartz Monzonite intruded, formed calderas. angle zone of shear beneath the batholith. The inferred feeder The Butte Quartz Monzonite to the north and east of the city zones for the Butte magma also fall along this linear zone. of Butte and the Rader Creek Granodiorite pluton to the south- Based on the east and west faulted margins of the Butte east intrude the Elkhorn Mountains Volcanics. Quartz Monzonite and its overall bilateral symmetry across a northeast-trending line, we suggest that the pull-apart bound- Butte Mining District ary faults likely root in the inferred northeast-striking fault zone beneath the Butte Quartz Monzonite. An implication of Despite the greater intensity of faulting in the Butte min- the bilateral symmetry and “rooting” interpretation is that the ing district relative to elsewhere in the batholith, the orienta- pull-apart grew from the bottom up. East and west spread- tions of the fault-veins are similar to the predominant fracture ing of the pull-apart could only happen in response to lateral trends observed throughout the batholith (for example, fig. displacement on the underlying northeast-striking basement 10). This similarity suggests that joints representative of shear zone. This shear was, in turn, a response to different released stored regional strains in the Butte Quartz Monzonite rates of displacement on the east-west–striking faults bound- had some control on development of the mineralized Butte ing the Helena embayment as the thrust front moved asym- fracture networks. metrically east in response to east-west compression. This Our suggested coupled strike-slip–splay fault model for model differs from other interpretations that suggest a pull- the formation of the different fault-vein sets in the Butte dis- apart driven by basal thrust detachment (for example, Schmidt trict has two important implications: and others, 1990; O’Neill and others, 2004). One or more pre-batholith décollements may have served as the base of the 1. Because extension is northeast-southwest between pull-apart, but they did not drive the structural systematics. northeast-striking strike-slip faults, fluid flow would A further difference is that our pull-apart driving mechanism have been effectively compartmentalized into gener- does not require the eastward displacement of the batholith. ally northeast-trending panels bounded by parallel Although geophysical data imply a somewhat broader zone northeast-striking faults. This possibility is consistent of early-stage plutons to the east of the Butte Quartz Mon- with the paucity of ore along the master strike-slip zonite than to the west (for example, fig. 13A), the pull-apart faults because there is little extension per se within very nearly bifurcated the early-stage batholith and the older these faults. plutons that now form a ring around the remarkably symmetric 2. Owing to the relatively low permeability of the north- Butte Quartz Monzonite. Though Schmidt and others (1990) east striking panel-bounding faults, hydrothermal hypothesize that the northeast-striking fault zone served as a fluid flow would have been predominantly vertical backstop to the pull-apart as the east margin moved east along within a panel. The hydrogeology thus implies that the trailing edge of a thrust, that hypothesis is not supported by the two types of polymetallic-vein mineralization either the seismic data (Vejmelek and Smithson, 1995) or the (silver-zinc-lead, Ag-Zn-Pb, and copper-arsenic-zinc, potential-field data. Instead, the geophysical evidence suggests Cu-As-Zn) did not form from the lateral flow of a that the early-stage plutons increase in volume with depth and single fluid with evolving composition. Two discrete are rooted at least as deeply as the Butte Quartz Monzonite. fluids best explain the spatial separation and compo- We suggest that it is because of the pinning of the thrusts by sitional differences. pluton emplacement that strike-slip faults became the domi- nant mechanism for accommodating compression during early-stage plutonism and provided space for the intrusions. Conclusions

Depth of Emplacement of Batholith The localization of the Boulder batholith plutons, dikes, veins, and fractures in the Great Falls tectonic zone, south- Houston (2002) argues on the basis of hornblende geo- western Montana, was influenced by a long history of inter- barometry in the Butte mining district that the Butte Quartz mittent deformation events involving the basement, some as Monzonite was emplaced 7–9 km below its present level old as 1,800 Ma. Intrusive and related volcanic activity began of exposure. However, it is highly likely that the top of the synchronously with superjacent thrust faulting, the fronts of plutonic rocks was in the range of a 2- to 4-km depth because which were guided by Proterozoic deformation zones. Shortly (1) vein textures in the Butte district more nearly reflect thereafter, the localization of plutons, dikes, and veins west of References 27 the active thrust front was controlled by strike-slip and related Cordell, L., Keller, G.R., and Hildenbrand, T.G., 1982, Bou- faults that likely represent reactivated Proterozoic faults. guer gravity map of the Rio Grande rift, Colorado, New Modeling of potential-field geophysical data using 2D and 3D Mexico, and Texas: U.S. Geological Survey Geophysical methods is consistent with an interpretation that the late stage Investigations Map GP-949, scale 1:1,000,000. of the batholith was emplaced in a binodal pull-apart structure probably related to the reactivation of an underlying Paleopro- Dudás, F.O., and Ispolatov, V.O., 1997, Nd, Sr and Pb isotopic terozoic suture zone-related fault. Asymmetric advance of the composition of the Lowland Creek Volcanics, west-central thrust front likely reactivated the suture-related fault owing to Montana [abs.], presented at Seventh Annual V.M. Gold- different rates of displacement on the east-west–trending north schmidt Conference, Tucson, Arizona: Lunar and Planetary and south boundaries of the Helena salient wherein the thrust Institute Contribution, v. 921, p. 62. faults were active. Eisele, J., and Isachsen, C.E., 2001, Crustal growth in south- The principal Proterozoic deformation fabrics are recog- ern Arizona—U-Pb geochronologic and Sm-Nd isotopic nizable in the orientations of fault-veins in the Paleocene Butte evidence for addition of the Paleoproterozoic Cochise block mining district. During the early Tertiary, northeast-striking to the Mazatzal Province: American Journal of Science, joints in the Butte Quartz Monzonite above the basement v. 301, p. 773–797. suture-related fault were activated through shear resulting in right-lateral faulting across the district. At segment boundar- Emsbo, P., Groves, D.I., Hofstra, A.H., and Bierlein, F.P., ies, that is where the progress of strike-slip displacement is 2006, The giant Carlin gold province—A protracted inter- interrupted, splay faults were formed along east-west and play of orogenic, basinal, and hydrothermal processes above northeast trends, and the right-extensional stepping of the a lithospheric boundary: Mineralium Deposita, v. 41, master northeast-striking faults resulted in a complex but p. 517–525. systematic array of faults within which the Butte hydrothermal activity was localized. Flodin, E.A., and Aydin, A., 2004, Evolution of a strike-slip fault network, Valley of Fire State Park, southern Nevada: Geological Society of America Bulletin, v. 116, p. 42–59. Acknowledgments Foster, D.A., Mueller, P.A., Mogk, D.W., Wooden, J.L., and Vogl, J.J., 2006, Proterozoic evolution of the western mar- We thank Karl Kellogg and Jeff Wynn for their insightful gin of the Wyoming craton—Implications for the tectonic critical reviews. Editor Melanie Parker provided much needed and magmatic evolution of the northern Rocky Mountains: counsel, as did Tom Judkins, and Sharon Powers provided cre- Canadian Journal of Earth Science, v. 43, p. 1601–1619. ative visual information assistance. This work was supported by the U.S. Geological Survey Mineral Resources Program. Grauch, V.J.S., and Cordell, L., 1987, Limitations of determin- ing density and magnetic boundaries from the horizontal gradient and pseudogravity data: Geophysics, v. 52, References p. 118–121.

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For more information concerning this publication, contact: Center Director, USGS Crustal Geophysics and Geochemistry Science Center Box 25046, Mail Stop 964 Denver, CO 80225 (303) 236-1312

Or visit the Crustal Geophysics and Geochemistry Science Center Web site at: http://crustal.usgs.gov/ Berger, B.R., Hildenbrand, T.G., and O’Neill, J.M.— Control of Precambrian Basement Deformation Zones on Emplacement of the Laramide Boulder Batholith and Butte Mining District, Montana, U.S.—U.S. Geological Survey Scientific Investigations Report 2011–5016